Consultant to VVorld Cold Council and Technical Editor, Cold Bulletin
'Newlands; The Village, Whitchurch Hill, Reading, RCB 7PN, UK
Some of the unique features of gold catalysis were described in Part 1. Further examples of reactionscatalysed by supported transition metal oxides are given in this second part. These include both selective andcomplete oxidation of hydrocarbons and a number of hydrogenation reactions. In fact, gold has now beendemonstrated to be the element of choice for some catalytic reactions. As a result, both chemical processingand environmental applications are foreseen for supported gold catalyst systems. In this, the second of twoarticles, the overall current state of knowledge on gold catalysis is reviewed and some early examples ofhomogeneous gold catalysis, in solution, assessed.
Haruta (3) has shown (Figure 1) that catalyticactivity, as expressed by temperature at 50%conversion, for the oxidation of methane (CH4)
* The material for this article is based on the second part of a talk presented
at the IPMI Annual Meeting in Toronto, 17 June 1998 which has now been
published as part of the Precious Metals 1998 'Proceedings of the
Conference held in Toronto, Ontario, Canada'.
Figure 1 Temperature for 50% conversion in the catalytic
Waters et a1 (2) have evaluated co-precipitated gold ontransition metal oxide catalysts for their activity towardsmethane oxidation to carbon dioxide and water.Activities were found to fall in the following order:
5% AU/C0304 is active at below 523 K. A dual sitemechanism was proposed to explain the results sincethe catalyst systems are also active at highertemperatures. The activities increased with increasedoxidation state of gold.
HETEROGENEOUS CATALYSIS
We have seen from the work described in Part I (1)that gold species supported on activated carbon are themost active for the hydrochlorination of acetylene(ethyne}, and that Au/a-Fez03 is the best catalystfound to date for the oxidation of carbon monoxide atlow temperatures. The control of gold particle size toca 2-5 nm is important and this can be achieved usingdeposition precipitation or co-precipitation preparativeprocedures for the catalysts.
In Part II of this pair of articles we will considerother heterogeneous reactions catalysed by gold,including catalytic combustion of hydrocarbons,selective oxidation, hydrogenation of carbon oxides,the reduction of nitrogen oxides with propene, carbonmonoxide or hydrogen, and the oxidativedecomposition of halogen compounds.
12 <:00' Cold Bulletin 1999,32(1)
propane (C 3Hs), and propene (C 3H6) decreases in theorder:
AulC030 4 > AuINiFez04 > Au/ZnFez04 > Au/Fez03(2)
In the oxidation of methane and propane thesupported gold catalysts are more active than thePt/Alz03 catalyst. The most highly active gold catalyst,ie AU/C0 304 gives approximately the same level ofactivity as a commercial PdfAlz03 catalyst. Goldshould therefore be considered along with palladiumand platinum for the complete oxidation of saturatedhydrocarbons. For the complete oxidation ofunsaturated hydrocarbons, such as propene, platinumand palladium are more active than supported goldcatalysts. Figure 1 also includes conversions fortrimethylamine, an indicator for the removal of odoursfrom gas streams.
Selective OxidationHaruta (4) found that for the oxidation of propene topropene oxide, greater than 99% selectivities could beobtained at low conversions (1%) using 1wt% Au/TiOzat 323 K. The feed gas was hydrogen / oxygen /propene / argon in the ratio 10 : 10 : 10 : 70 vol% at aflow rate of 2 x 103 ml n', using 0.5 g catalyst. Thecatalyst was prepared by deposition precipitation.Similar palladium and platinum catalysts produced nopropene oxide under similar conditions. As with goldcatalyst systems used in complete oxidation reactions,the method of preparation was again found to beimportant and best results were obtained when the goldparticles had a mean particle diameter of 2.4 nm and anarrow size distribution. More research is now requiredregarding means for achieving high selectivity at higherconversions, although a recycle process using a lowconversion situation could also be considered.
Hydrogenation ofCarbon Oxides
If gold supported on zinc oxide is prepared by coprecipitation, it gives methanol from the hydrogenationof carbon monoxide and carbon dioxide (4), while othernoble metal catalysts give methane as the principalproduct. The selectivity and activity differ only a littleamongst the Group 11 metals; basic metal oxides such asZnO are the best for production of methanol, whilstacidic metal oxide supports, eg TiOz, give carbonmonoxide via reverse water gas shift at much higher rates.In Figure 2 the methanol yield is given as a function ofthe reaction temperature in the hydrogenation of carbondioxide, for a number of supported gold catalysts. The
(fji;} Cold Bulletin 1999,32(1)
6
Cf(S
:9 4.9<>-,
3"0>::oJ
2.;;<ll
;S
0373
Figure 2 Methanolyield as a function ofreaction temperature
in the hydrogenation ofcarbon dioxide over supported
Tt) = 1:19. CO2.H2.Ar =23:67'10; 50 atm pressure;SV= 3000 ml {g cat}-l u'. (Based on reference 4)
s.----....- ....--,--r---,..--...,
4
<ll 3
""P::: 2
733
Figure 3 Temperature dependence ofthe catalytic activity of
supportedgold and base metal oxide catalystsin theV16ter Cas Shift. Starting reaction gas mixture was
4.88 vol% carbon monoxide in argon; water vapourpartialpressure223 Torr;SV4000 u:': 1atm. AulOI.
Fe203 (0); CuO/ZnO/A1203(O);0I.-Fe203 (e);AulA1203 (.). Rates are expressedin mol m-2 k 1 x10-2. (Based on reference 6)
activity per unit surface area increases with decreasingparticle diameter of the gold (5).
The use of Au/a-Fez03 in the catalysis of thewater gas shift has been reported by Andreeva et a1 (6,7) (Figure 3). The gold particles had an average size of3.5nm and were situated in close proximity to the ironoxide crystallites. The gold on iron oxide catalyst wasactive at low temperatures, and its performance overallcompares favourably with the CuO/ZnO/Alz03catalyst which is commercially well established for thisreaction. Haruta et a1 (8) have also reported catalystsactive at low temperatures: gold supported on titania,
13
prepared by deposition precipitation of gold hydroxidefrom chloroauric acid and titanium dioxide powder,was shown to be active for both the forward and thebackward water gas shift.
Reduction ofNitric Oxide with CarbonMonoxide, Hydrogen or PropeneThe reduction of nitric oxide with carbon monoxide inthe absence of oxygen occurs at temperatures below373 K over gold supported on a-Fez03 or NiFez04,yielding nitrogen as the main product, while theproduct over unsupported gold is NzO (4).Au (I)/ZSM-5 is also active for this reaction at lowtemperatures (9). Platinum Group Metals requirehigher temperatures and produce more nitrous oxide(4). The reaction between nitric oxide and carbonmonoxide is deactivated by the presence of oxygen, butnot by water (over Au/a-Fez03)'
Salama et al (10 - 13) have studied the reduction ofnitric oxide by carbon monoxide and hydrogen:
2 NO + CO + Hz ---.,. N, + COz + HzO (3)
Freshly reduced Au/NaY zeolite catalyst was used at 673K and some of the results are illustrated in Figure 4.
The reduction of nitric oxide to nitrogen withpropene takes place over several supported goldcatalysts and is accelerated by the presence of oxygenand water (14): Figure 5 indicates that gold on zincoxide, magnesia, titania, or alumina is active between473 and 773 K. Au/Alz03 gives the highest conversionof nitric oxide to nitrogen. Over the Au/AIZ03 catalystnitric oxide is oxidized to nitrogen dioxide which thenreacts with propene to give nitrogen. The addition of
2540
Cf( ~>:: 20";;.S .Si:3 i:3<ll <ll> >>:: 15 §0u u
0 0Z u
105 10 15 20 25Time-an-stream (h)
Figure 4 Conversion ofnitric oxide and carbon monoxide to
nitrogen and carbon dioxide respectivelyas functions
Figure 5 Temperature dependence ofnitric oxide conversion to
nitrogen using propene for Au/ZnO (0); Au/fl02
(L,); Au/MgO (0); Au/AI203 (0); and A1203(.). Cold loading is 1.Owt%; reaction gas is nitric
oxide 1000ppm, propene 500ppm, 50 vol% oxygen,
1.8vol% H20, He balance; space velocity 2x104ml{g cat} -1 Ii:', (Based on reference 4)
MnZ03 to the Au/alumina in order to enhance nitrogendioxide formation, markedly improves the conversionof nitric oxide to nitrogen over a wide range oftemperatures (15). This composite gold catalyst offersone of the best performances in nitric oxide conversionamongst the catalysts so far developed (see also A. Uedaand M. Haruta, Gold Bull., 1999, 32, 3 -this issue).
Reactions with Halogenated CompoundsGold supported on cobalt oxide or alumina areclaimed to be as active as platinum catalysts for theoxidative decomposition of dichlorodifluoromethaneand methyl chloride (16). For example, 5wt%AU/C0304 can be used to completely decomposemethyl chloride at around 600 K.
Gas purification
In addition to the removal of carbon monoxide from airand other gas mixtures, other gas cleaning roles can beenvisaged for supported gold catalysts, eg hydrogen hasbeen removed from carbon dioxide feed gas, used in theproduction of urea, using gold supported on iron oxideprepared using a co-precipitation technique (17).
It has gradually emerged over the years that carefulpreparation of gold and gold alloy catalysts is a key toobtaining repeatability in their performance. In describing
<:00' Cold Bulletin 1999,32(1)
the activity of bimetallic catalysts in 1985, Schwank(18) concluded that "The nature of the support andthe preparative conditions play a decisive role indetermining the microstructural characteristics of theactive catalyst surface. In many cases gold seems to actmore or less as an inert diluent, and ensemble orcluster effects seem to dominate the catalyticbehaviour. These ensemble effects can be used tomanipulate catalytic activity".
The importance of careful preparation has beenemphasized much more recently by two of the leadersof research in heterogeneous gold catalysis, Haruta (4)and Hutchings (19) . Haruta has described theadsorption properties and reactivities of gold in termsof their size dependency from bulk to fine particles.
The catalytic performance of gold markedlydepends on dispersion, supports and preparationmethods. When gold is deposited on selected metaloxides as hemispherical ultrafine particles withdiameters smaller than 5nm, it exhibits surprisinglyhigh activities and/or selectivities in the combustion ofcarbon monoxide and saturated hydrocarbons, theoxidation/decomposition of amines and organichalogen compounds, the partial oxidation ofhydrocarbons, the hydrogenation of carbon oxides,unsaturated carbonyl compounds, alkynes andalkadienes, and the reduction of nitrogen oxides. Inboth the selective oxidation of propene and thecomplete oxidation of carbon monoxide, gold particlesizes in the range 2 - 5 nm give the highest turnoverfrequencies and product yields (4).
Hutchings (19) emphasized the importance ofHaruta's work, confirming that for low temperaturecarbon monoxide oxidation the method used forcatalyst preparation was crucial. Co-precipitation anddeposition precipitation have been shown to be thebest preparative methods, producing hemisphericalparticles strongly attached to the oxide support.Haruta showed that gold catalysts prepared using aconventional impregnation procedure are considerablyless active than platinum group metals catalystsprepared in a similar way, but if a co-precipitationprocedure was utilized, high activity catalysts wereobtained with gold. The catalysts were prepared fromaddition of an aqueous solution of chloroauric acidand a metal nitrate to an aqueous sodium carbonatesolution. The precipitate was then washed, vacuumdried and calcined at 673 K. In addition, Haruta (20)has also shown that catalysts can be prepared usingdeposition precipitation. A uniform dispersion of smallgold particles on the support is a necessarycharacteristic for good performance. The best results
(fji;} Cold Bulletin 1999,32(1)
are obtained when reaction temperatures are >328 K(4). Direct comparison with other precious metals isnot necessarily available because impregnation is themost commonly used technique for these metals.
The influence of preparation methods on thecatalytic activity for carbon monoxide oxidation wasfound to be very large for Au/TiOz and negligible forPt/TiOz catalysts (21). Platinum and gold weredeposited on titania by deposition precipitation,photodecomposition and impregnation. Thedeposition precipitation method gave the most activecatalysts for both platinum and gold. Gold catalystsprepared by deposition precipitation were active attemperatures below 273 K and showed a much greateractivity than platinum catalysts.
It is clear that gold dispersed on transition metaloxide has an intimate interaction with the support.Gold alone is less active as a catalyst than supportedgold. Until recently much of the literature on the topichas assumed that metallic gold is the active species, butthe present reviewer would like to highlight thepossibility of a monolayer of gold oxide being presenton the surface of the gold during oxidation reactions(22). The interface between the gold and the supportand the interaction between the gold and the supportalso appear to be important (21). The activities areaffected by the pretreatment procedures and recentwork indicates that higher activities are obtained if thesupported gold is not calcined (23 - 25).
HOMOGENEOUS CATALYSIS
Involvement of gold complexes of any type inhomogeneous catalysis is very rare, and there are only alimited number of known examples. The rarity ofexamples is in marked contrast to those for theplatinum group metals, many of which readilyundergo catalytic oxidative-addition / reductiveelimination cycles (26). This has been rationalized bysaying that this type of catalysis requires a very delicatebalance between the stabilities of the two oxidationstates involved, and this has not often been achievedfor gold. An additional factor is thought to be thereluctance of gold to form hydride complexes, so thatthe oxidation of gold (I) by dihydrogen, or theformation of alkene complexes by f)-elimination ingold(III)-alkyl complexes are virtually unknown (thereis one case recorded of a teFt-butyl iso-butylisomerization, thought to occur by f)-elimination(27)). Although goldtl) complexes readily undergooxidative addition, eg with alkyl halides, the resulting
15
complexes have appeared to be either too stable toreact further or so unstable that they react immediately.Reductive-elimination from di- or tri-organogold(III)complexes is either restricted to particular types oforganogold complex or requires robust treatmentresulting primarily in elimination of hydrocarbons bycoupling reactions. For other precious metals, therelative stabilities of the two critical oxidation stateshave been successfully adjusted by appropriate choiceof ligands, egthe inclusion of a good n-bonding ligand,
such as carbonyl, increases the stability of the loweroxidation state. In the case of gold n-bonding hasseemed to be of relatively little importance.
The status for homogeneous catalysis by gold has,however, been dramatically transformed by the recentlyreported results of Teles et a1 (28). This BASF grouphave described the use of cationic gold (I) complexes ofthe type [L-Au+] (where L is a phosphine, phosphite oran arsine) for the addition of alcohols to alkynes. Anexample of such a reaction is given below:
The turnover numbers for this type of reaction werefound to be up to 105 moles of product per mole ofcatalyst, with turnover frequencies of up to 5400 h-1.
These gold catalyst systems are a significantimprovement on the mercury catalysts used previouslyand the reactions are conducted under mild conditions(293 - 323 K) in the presence of acid co-catalysts.
Carbonylation of Olefins:
Olefins + CO - t-RC02H
Catalyst:
H2S04 COAU203-Au
3+(solv) -Au+(solv)
CO +CO- [Au(CO)f~ [Au(COhf
-CO
Figure 6 Catalyst for the carbonylation ofolefins in sulfuric
acid (Based on reference 29)
CH3/ MeOH
CHz-C==C-CHz -7I 55°C
H 3C
MeO OMe
\/H 3C C CHz (4)
\/\/'\CHz CHz CH3
CO
R2
1 IR-C.
I >-- +Au(CO)CH 2
[
R' - ~>"')'.o ]CH2-A~o
'co
CO
+CO~ AU+(COh-CO
CO coAu3
+~ Au' _ Au' (CO)
CO2
Figure 7 Proposed mechanism for the carbonylation ofolefins using a soluble gold catalyst. (Based on reference 29)
16 <:00' Cold Bulletin 1999,32(1)
Figure 8 Proposed mechanism for Komiya condensation
reaction. (Based on references26 and 30)
A chemical description of a gold carbonyl catalystis given in Figure 6 (29). Gold(III) oxide (AUZ03) inconcentrated sulfuric acid is reduced by carbonmonoxide to form [Au(CO)nJ in one step. The goldmonocarbonyl (1) and the gold dicarbonyl (2) coexistin this solution. This non-classical gold (I) carbonylsolution has proved to be an effective catalyst for theformation of t-carboxylic acids from olefins at roomtemperature and atmospheric pressure but the turnovernumbers are very small. The proposed mechanism isgiven in Figure 7. This is an example of a Kochreaction, also known to be catalysed by copper andsilver.
Gold (I) and gold (III) alkoxides have been shownto catalyse the condensation of benzaldehyde withcompounds containing an active methylene group,CHz(X)Y, again with very low turnover numbers (26,30):
MeI
L-Au-MeIOR
IV
CH,(X)Y ROH
MeI
L-Au-MeICH
f\V
MeI
L-Au-MeIoHp-;.:h
X~c\y
PhCHO
R2
_ / Au catR'--C=C- (CH2)3-C~
NH2
Best results were obtained with goldll) catalysts of themore electron-withdrawing alkoxides: R3PAu(OR'), R= Ph, C6H ll ; R' = OCHz(CF3)z. The proposedmechanism is shown in Figure 8: the alkoxide IVinitially reacts with CHz(X)Y from which it abstracts aproton to form the corresponding alcohol and an AuCH(X)Y species V (such compounds themselvescatalyse the reaction, but less efficiently). The carbonylgroup of benzaldehyde inserts into the Au-C bond ofV and the resulting complex deprotonates eitherCHz(X)Y or the alcohol, to reform IV or V; theinitially formed alcohol (VI) loses water spontaneously.
Another useful example of homogeneous catalysiswhich appears to involve an organogold intermediate isthe cyclization of alkynyl amines such asRIC=C(CHz)3CHRZNHz (Figure 9) catalysed byNa[AuCI4J (31). It was suggested that a cyclicintermediate is formed by exo-dig addition of Au andN across the triple bond which then undergoesprotonolysis and isomerization. The reaction mixturerapidly decolourizes, implying the involvement ofgold (I), and the most likely mechanism would be via
initial formation of a n-alkyne complex. Thecoordinated triple bond would then be open tonucleophilic attack by the amine group; undersufficient dilution this would occur intramolecularly.
The 'Hyashi' ferrocenyl gold catalyst which is
(fji;} Cold Bulletin 1999,32(1)
Ar~COOMeAu cat I \
+ C=:NCH2COOMe - 0 N-0
Figure 9 Examples ofhomogeneous catalysis using gold catalysts.
(Based on references26, 31 and 32)
achieving prominence for the synthesis of asymmetricheterocycles (Figure 9) is also a goldll) complex,although its mode of action involves formation of anisonitrile complex of gold, rather than anorganometallic intermediate (32). There is someevidence that [AuCI 4J- catalyses the addition ofnucleophiles such as amines and alcohols to isonitriles(33) and that the mechanism involves the formation ofcarbene complexes (34), but in all cases copper(1) is afar better catalyst.
The possibility of commercial relevance for goldhomogeneous catalysis is indicated by the publicationof patents by Enichem SPA, Italy naming Calderazzoet a1 as inventors (35). These patents claim ahomogeneous catalyst system for the preparation ofolefin (ethylene, a-olefin) / carbon monoxidealternating copolymers. The catalysts consist of at leastone Group 11 metal salt (Cu, Ag, Au), a P- or Ncontaining bidentate chelating base, and an oxidizingagent (nitrosium tetrafluoroborate or p-benzoquinone)
17
which is not hydrophilic. The process is used for copolymerizing ethylene and/or other olefinicmonomers, either as individuals or as a mixture of twoor more, with carbon monoxide. The method simplyinvolves mixing the components together in a solvent.Appropriate mixtures of olefins can be employed to'tailor-make' polymer properties such as melting point,glass transition temperature and processability.
SUPPORTED HOMOGENEOUSCATALYSIS SYSTEMS
There are some examples of homogeneous catalystswhich are known to catalyse reactions after beingsupported on oxides. For example, Schmid (36) haspractical evidence for the importance of 55 atomclusters (particle diameter 1.4 nm) and has describedthe preparation of AU55C16(PPh3) iz- The isomerizationof hexanes (2,2-dimethylbutane to rz-hexane) wasstudied using 1% AU55 on TiOz (anatase). Au/Pdcolloids, in which the gold is coated with palladiumshow enhanced catalytic activity for the hydrogenationof 2-hexyne to 2-hexene, and palladium coated withgold is also active, even though pure gold is inactive.
A phosphine-stabilized mononuclear goldcomplex, AU(PPh3) (NO 3), and a phosphine-stabilizedgold cluster, [Au9(PPh3)S](N03)z, have been used asprecursors for supported gold catalysts. Bothcomplexes were inactive for carbon monoxideoxidation if supported on oxides such as a-Fez03,titania and silica, whereas the compounds supportedon oxides when treated under air or in carbonmonoxide or in 5% H z/Ar atmosphere were found tobe active for carbon monoxide oxidation. The catalyticactivity depends not only on the conditions oftreatment but also on the kind of the precursor and thesupports used. The catalysts derived from the firstcomplex showed higher activity than those derivedfrom the second. a-Fez03 and titania were much moreefficient supports than silica for the gold particles(37, 38).
Pignolet has studied catalytic applications ofplatinum- and palladium-gold clusters. The Hz/Dzequilibrium was investigated using[Pt(AuPPh3)S] (N03)z. There was a rate-determiningdissociation of PPh3. The clusters were then supportedon alumina and silica and the Hz/Dz equilibriumagain studied. If activated at moderate temperatures,the cluster could be desorbed after catalysis. It wasconcluded that partial and reversible ligand desorption
18
and/or cluster distortion takes place on the supportgiving rise to significant effects on activation.Treatment under hydrogen increases this effect. MagicAngle Spinning solid state NMR was used tocharacterize the surface species (39).
CONCLUSIONS
Present knowledge confirms that catalysis by gold isdistinctly different from catalysis using other preciousmetals, both in the techniques required to prepare thecatalysts and in the ways that they function duringreactions, ie some aspects are unique to gold (1). Thereare a number of areas where gold catalysis has potentialfor applications, both in the environmental controland chemical processing areas. In depth study of goldcatalyst technology is likely to lead to a betterunderstanding of the effects so far recorded and shouldprovide a sound basis for the use of this technology inthese fields.
Gold catalysts could have promise for low light offapplications, but sintering problems may have to besurmounted at higher temperatures, where for exampleplatinum (m p 2042 K) has the advantage ofpossessing a much higher melting point than gold (m p1337 K). As a result, the Tammann temperature, atwhich the metal particles begin to sinter is significantlylower for gold. Strong bonding between the gold andits support could, however, overcome this potentialdisadvantage. For removal of noxious gases such ascarbon monoxide and nitrogen oxides supported goldalso has potential, and the removal of hydrocarbons isa distinct possibility.
The potential of supported gold catalysts forhydrogenation of carbon oxides and for catalysis of thewater gas shift has already been examined and couldwith benefit be further developed. Homogeneouscatalysis has been studied by some industrial researchorganizations; and the dramatic recent breakthroughusing gold catalysts in solution for the addition ofmethanol to triple bonds could eventually have farreaching consequences. As a result, the hypothesespreviously put forward to rationalize the ineffectivenessof gold as a homogeneous catalyst will need to berevised.
As we have seen, gold was previously regarded asan inferior catalyst compared with the other preciousmetals (1), but the examples given in this pair ofarticles clearly demonstrate that there are a growingnumber of reactions where it can be the catalyst ofchoice. These examples encompass the fields of
<:00' Cold Bulletin 1999,32(1)
ABOUT THE AUTHOR
REFERENCES
1 DT Thompson, ColdBull, 1998,31, 111
2 RD. Waters, JJ WeimerandJE Smith, Gatai Lett, 1995,30,181
The author is grateful for supply of information by DrMasatake Haruta (Osaka National Research Institute,Japan) and for helpful discussions with ProfessorsGeoffrey Bond (BruneI University, Uxbridge, UK) andGraham Hutchings (Cardiff University, UK). He isalso indebted to Professor R V Parish (UMIST, UK)for information on homogeneous catalysis, receivedprior to publication. Figures 1-5 were re-drawn fromthe original literature during preparation for a reviewarticle entitled 'Catalysis by Gold' by G C Bond andD T Thompson and recently accepted for publicationin Catalysis Reviews - Science and Engineering. Thanksare also due to Dr Lucy Thompson for literaturesearches and to the IMPI for permission to publish thewhole of the Toronto talk (see footnote to p 12).
33 T Saegusa, Y Ito, S. Kobayashi, K Hirota and H Yoshioka, Bull Chem. Socjapan,
1969, 42, 3310
34 JE Parkes andAL BaLch,] Grganomet Chem, 1974, 71, 453
35 F Caiderazzo, F Garbassi, G. Lugii and A Sommazzi, European Patent 200695
930311,1993 (EP560455 AI, EP 560456 AI)
36 G. Schmid, 'Progress in the Science andTechnoiogy of Goid',Hanau,June 1996,
seeColdBull, 1996, 29, 105
37 Y Yuan, K Asakura, H Wan,K Tsaiand Y Iwasawa, Catai Lett, 1996, 42, 15
38 Y Yuan, AF Koziova, K Asakura, H Wan, KTsai and Y Iwasawa, j Catai, 1997,
170,191
39 L. Pignoiet, 'Progress in the Science and Technoiogy of Goid', Hanau, June 1996, see
ColdBull, 1996, 29, 70, 105
13 G. R Bamwenda, A. Obuchl,A. Ogata,J o,S. Kushiyama and K Mizuno,]
Mol Gatai, kChemical, 1997, 126, 151
14 A Uedaand M Haruta, AppL Gatai B:Environmental, 1996, 285,81
15 A Uedaand M. Haruta,Appl. Gatai B:Environmental, 1998, 18,453
16 T Aida,R Higuchiand H Niiyama, Chem Lett, 1990, 2247
17 Z. Hao, LAn, J Zhou andH Wang, React Kin CataI. Lett, 1996, 59, 295
David Thompson has participated in and managedupstream, basic and applied research activities inuniversities (Imperial College, London and Universityof California, Los Angeles), ICI, and Johnson Matthey.His published papers are concerned with catalysis andmaterials topics, particularly those involved withprecious metals, as well as with organic and inorganicchemistry. He is Technicel Editor of Gold Bulletin andConsultant to World Gold Council. In his freelancework as a chemical consultant he advises on catalystsystem design for chemical processing, pollutioncontrol and gas detection. Some of this work isperformed in collaboration with universities, and hehelps industrial organizations to optimize their contactswith university research. He has contributed to andassisted in the editing of a new book to be published in1999 entitled 'The Company of the Future'.
